The thing to know is that what goes to Convert and Display can be just loosly related to row cells. Basic AddColumn starts with "single Value", however you can Add more indicies to the column, in that case you will get ValueMap instead of single Value in Display.
So basically, you need to AddIndex (which does not add column), then for each AddColumn add this index. In Display you will get ValueMap of column value combined with this 'index'.